Yes, if you master one, then there is no conflict. But my son was a monolingual person learning another language. And he kept trying to apply Russian ideas to

Yes, if you master one, then there is no conflict. But my son was a monolingual person learning another language. And he kept trying to apply Russian ideas to Chinese, and Chinese ideas to Russian. Therefore, even though he personally wanted to know Russian more, he chose Chinese instead because it met a school requirement for a second language. (And they couldn't teach him Russian.) He still plans to learn Russian, though.